464XLAT support for mobility

You can specify the IPv6 prefix length for the CLAT source address using the new command clat-ipv6-prefix-length. When you configure this command, NAT rules apply 464XLAT based on the destination-address of the traffic, and source-address and source-prefix are no longer required. The clat-ipv6-prefix-length command is available at the [edit services nat rule rule-name term term-name then translated] hierarchy level.